It’s almost always the case that your drill hole will be wider than
the saw cut made by the saw you can get into a given hole. But if
the saw blade you’re using is a 4/0, you may be able to use two
steps. Start with a number 80 drill, about the smallest made.
rocking it back and forth in a line can drift the hole slightly (be
careful not to break the drill), till you can get an 8/0 blade into
that hole. Now use the 8/0 blade to start the cut, and by going over
it a couple times, you can widen the cut to the full width of the
original hole. Now, since the cut is as wide as the drill hole, but
longer, you’ll be able to fit in your larger, 4/0 blade.
Another method that can sometimes be used is to start with the
smallest drill hole that will allow your desired blade, and cut a
ways away from the hole, remove the blade, and with a very small and
light hammer, gently planish the area just to each side of the hole.
The metal will slightly stretch, compressing towards the hole and
closing up the saw kerf there. You can then saw back through it,
since you’ve got the ends of the saw cut where you can still fit
your blade, and when you saw back through that section, you’ll remove
the traces of the drill hole. But this assumes you can tolerate the
slight distortion caused by that light planishing…